The present invention relates to a transmission control method for a radio communication system and, more particularly, to a communication system which performs SDL (Super high speed Down Link) transmission in which down-link transmission can be performed through a radio channel at a higher speed than up-link transmission.
With the widespread use of compact, portable data processing and electronic instruments, communication functions have been added to these portable instruments, and services using various networks have been provided.
In general, as a communication means to be applied to a portable electronic instrument, a communication means allowing a user to easily communicate any information with anybody at any time in any place is desirable in consideration of the merits of portability.
As such a communication means, it is best to use a radio communication system such as a PHS (Personal Handyphone System; convenience portable telephone), or a mobile communication system (portable telephone or a mobile telephone system), which can be connected to a public network and for which many radio base stations are distributed. This system allows transmission/reception of data by radio communication between the radio base stations and the radio terminals within the service areas of the respective radio base stations.
In such a radio communication system, when a radio terminal transmits a communication request signal, the request signal is supplied to a radio base station through an up-link radio channel. With this operation, the radio base station assigns a communication channel to the radio terminal, so that the radio terminal can communicate with a destination terminal by using the communication channel through the radio base station. The radio terminal transmits data to the radio base station through an up-link radio channel. Data is transmitted from the radio base station to the radio terminal through a down-link radio channel.
In a radio communication system of this type, when a multimedia service such as picture communication as well as simple speech communication is to be realized, the amount of data greatly increases. In order to handle video picture data, a large amount of data must be transmitted within a short period of time. For this purpose, a wide-band channel is required. This means that high-speed transmission is required in terms of transmission speed in communication.
When a multimedia service such as VOD (Video On Demand) or digital electronic publication (newspapers, magazines, and books in electronic forms), in particular, is to be provided, the amount of data transmitted through a down-link radio channel is much larger than the amount of data transmitted through an up-link radio channel which is used to transmit only a request and an Ack message (acknowledgment message). For this reason, transmission must be performed at a much higher speed through a down-link radio channel than through an up-link radio channel.
This is because, small-volume data such as speech data and data for requesting information are transmitted through an up-link radio channel, whereas large-volume data such as picture and text data are transmitted through a down-link radio channel.
In a conventional radio communication system, however, the transmission speed in an up-link radio channel is equal to that in a down-link radio channel. That is, the transmission speed of a signal transmitted from a radio terminal is set to be equal to that of a signal received by the radio terminal.
According to the conventional idea of a system configuration, in order to realize a multimedia service in a radio communication system, the system must be designed to perform high-speed transmission between a radio base station and a radio terminal by using wide-band channels as up- and down-link radio channels.
However, high-speed transmission demands a radio wave in a high-frequency band, and hence very high power. In general, a portable instrument as a terminal side uses a battery as a power source. With an increase in power consumption, therefore, a recharging operation must be frequently performed on the terminal side, resulting in a deterioration in operability. As is apparent, the battery capacity of the terminal may be increased. If, however, the battery capacity is increased, the size and weight of the instrument increase as well as the cost.
An SDL system (Jpn. Pat. Appln. KOKAI Publication No. 6-137621) has been proposed as a radio communication system which solves such problems. In this SDL system, in order to reduce the power consumption, no wide-band transmission section (transmission section with high transmission speed) is arranged on the radio terminal side, and wide-band communication is limited to a reception function while transmission is performed as narrow-band communication (low transmission speed) with low power consumption. With this arrangement, up- and down-link communications can be performed while a decrease in power consumption of the terminal can be attained.
As described above, in order to realize a multimedia service by using a radio communication system, high-speed transmission must be performed between a radio base station and a radio terminal through a wide-band radio channel. However, high-speed transmission demands radio waves in a high-frequency band, and very high power.
As a technique for solving this problem, the SDL system has been proposed. As described above, in the SDL system, in order to reduce the power consumption, no wide-band transmission section (transmission section with high transmission speed) is arranged on the radio terminal side, and wide-band communication is limited to a reception function while transmission is performed as narrow-band communication (low transmission speed) with low power consumption. With this arrangement, up- and down-link communications can be performed while a decrease in power consumption of the terminal can be attained.
The narrow-band communication function may include a down-link (reception) channel as well as an up-link (transmission) channel, and battery supply control may be performed such that communication which does not require high-speed transmission is performed by using narrow-band up- and down-link radio channels, while the power to the wide-band reception section of the terminal is turned off. With this control, a further reduction in power consumption of the terminal can be attained.
If the narrow-band up- and down-link radio channels are used not only as radio channels for data transmission but also as channels for controlling data transmission using the wide-band down-link radio channel, efficient data transmission using the wide-band down-link radio channel can be realized.
No transmission control procedure, however, has been proposed, by which narrow-band up- and down-link radio channels are used for efficient data transmission using a wide-band down-link radio channel.
In the SDL system, since different transmission distances and different allowable error rates are required for transmission of narrow-band and wide-band signals, it is conceivable that the frequency of a radio carrier used for transmission of the narrow-band signal is lower than that used for transmission of the wide-band signal. In practice, when a radio transmitter for transmitting narrow-band signals is formed, harmonic components having frequencies higher than the frequency of a signal oscillated by an oscillator and used as a carrier transmitted from a modulator is generated owing to the nonlinear characteristics of the mixer, the power amplifier, and the like. These harmonic components are attenuated by the filter function of the duplexer (transmission/reception shared unit) so as not to be radiated from the antenna.
In an apparatus having a structure in which a narrow-band radio transceiver and a wide-band radio receiver are incorporated in the same housing, even if the above harmonic components are not radiated from the antenna, harmonic components may be received by the antenna or the low-noise amplifier, interconnections for connecting these components, or the like on the wide-band radio receiver side owing to electric or magnetic coupling, leakage, or the like in the housing.
If such a harmonic component has the same frequency as that of a carrier at the radio section or intermediate-frequency section of the wide-band radio receiver, a received wide-band signal may not be properly demodulated. Conventionally, when devices susceptible to electric or magnetic coupling are to be arranged nearby, in order to solve such a problem, the devices are isolated with insulators and surrounded with a shield. With this structure, the two devices are isolated from each other to obtain a high attenuation gain.
For this reason, in a terminal having both a narrow-band transceiver and a wide-band radio receiver, the weight and volume of the terminal increase owing to an additional unit for providing such isolation.
A radio terminal currently has a data processing function, an advanced display function, and the like and hence can use various services using various networks. For example, various types of data such as speech, data, a still picture, and a motion picture are transmitted through a radio channel. In order to allow radio transmission of such data by using one radio terminal, an optimal transmission scheme must be provided in accordance with the type of data to be transmitted. In transmitting such data by radio, the following two schemes can be used: a scheme of multiplexing data with one modulating signal or carrier, and a scheme of using different carriers in accordance with the attributes of data.
An RF amplifier is one of the factors that interfere with a reduction in the size of a radio terminal which should be compact. In order to reduce the size, cost, and power consumption of the RF amplifier, different frequencies or transmission schemes suitable for different types of data are preferably used.
If, however, different frequencies or transmission schemes are used, some data can be transmitted by radio but other data cannot be transmitted in the same environment. In other words, the types and qualities of services which can be provided vary.
This is because, service areas greatly vary in accordance with frequencies used, and power consumption also varies. Consequently, services which can be provided vary in accordance with the battery residual capacity. In addition, the types, qualities, times, and the like of services which can be provided when two radio terminals receive a communication service are greatly influenced by the types, number, and qualities of radio signals which can be received by the distant terminal and its battery residual capacity as well as the types, number, and qualities of radio signals which can be received by the self-terminal and its battery residual capacity. Conventionally, however, the types, qualities, and times of services which can be provided cannot be determined in consideration of the state of the distant terminal.
If, however, a user cannot know specific services of various available services which can be currently used and the current state of each service, the user can determine whether he/she can use a desired service only when he/she performs an operation for using the desired service. In addition, the user can determine whether a provided service is satisfactory or not, only after he/she uses the service. That is, the user uses services by trial and error, resulting in poor operability. Furthermore, the user must pay the charges for services. If, therefore, a service is not satisfactory, the cost is wasted. That is, it is uneconomical to use such a service. Although various services can be used, the user cannot know which service can be currently used and its current state. For this reason, even if an environment that allows anybody to use a radio terminal with advanced functions anytime is provided, anybody cannot always use the system easily, interfering with the popularization of the system.
Demands have therefore arisen for an easy-to-use radio system which allows users in various age groups to easily receive various services using radio terminals.